Detecting radioactivity in dynamic situations—where the radiation signal is varying with time—is problematic, if the goal is to detect the smallest true signal, and at the same time not generate a “detection” signal if there is no signal present. This time variance of the radioactivity signal can be for many different reasons. Common ones include:                small segments of radioactivity mixed with normal material moving down a pipe or conveyor belt past one or more fixed radiation detectors;        radioactivity in a vehicle moving past one or more fixed radiation detectors;        one or more radiation detectors moving past radioactive objects.        
The normal method of signal analysis is to establish a noise or background level for the signal, and then compare the signal during a pre-determined time window against this background noise level, using an appropriate statistical test. And then repeat this test for the next time window; etc.
The problem with this method is to determine the most appropriate time window. If it is made too wide, or too narrow, with respect to the time-varying signal, the detection levels will be not as good as they can be. Too wide a window includes more noise than optimum, and reduces the ability to detect small sources. Too narrow a window eliminates much of the signal, and reduces the ability to detect small sources.
A similar problem occurs if there are multiple sensors viewing an object where the signal from the object is not the same as viewed by each of the detectors. This can be because the radioactivity in the object is not uniform—some of it is closer to certain detectors than to other detectors, or obscured by more attenuating material between the radioactivity and the detectors. To maximize the ability to detect small elevations of radioactivity, the signals from multiple detectors are frequently combined [e.g. summed]. The problem is to determine the most appropriate combination of detectors to allow detection of the smallest amount of radioactivity present in the object. The objective is to only combine those detectors where their signal contribution is more beneficial than the harm caused by their noise contribution. Using too many detectors includes too much background noise, and reduces the ability to detect small sources; using too few detectors eliminates useful signal, and reduces the ability to detect small sources.
These two situations can also be combined; small elements of radioactivity in an otherwise non-radioactive object or matrix that are not in known or predictable locations, and where this matrix is moving past a group of fixed detectors, each one viewing the object or matrix from a different vantage point. It is now very difficult to make an a-priori estimate of the proper time window and detector grouping for the optimum summation to allow detection of the lowest level of radioactivity.
The same situation occurs in gamma spectroscopy where the nuclide decays with several different gamma energies. Under some conditions gamma line 1 might be the best one to use; under some conditions gamma line 2 might be the best one to use; and under some other conditions the combination of line 1 and line 2 might be the best one to use.
Any of these 3 measurement conditions might be combined; multiple signal widths vs. time; multiple signals from different detectors, and multiple energy windows within a single detector during a single time interval.
Accordingly, it is a principal object of the invention to provide a method to detect a signal in dynamic situations.
Other objects of the invention, as well as particular features and advantages thereof, will be apparent or be elucidated in the following description and the accompanying drawing figures.